We aim to better understand the impact of Intensive Care Unit (ICU) workload on patient outcomes, so that practitioners and researchers can use such understanding to provide high quality care despite increased hospital crowding. We use data collected from the medical ICU and the surgical ICU of a major teaching hospital. We measure ICU workload in a novel way that takes into account not only the census but also patient acuity. Having categorized the ICU workload at time of patient departure as low census, high-census/low-acuity, and high-census/high-acuity, we find that patients discharged on a day of high-census/high-acuity ICU workload had significantly worse health status than patients discharged from a low-census ICU workload. Moreover, we find patients with poorer health status at the time of ICU discharge experienced worse longer-term outcomes, including longer post-ICU length-of-stay (LOS), higher mortality, and higher total hospital costs. In sum, we find that acuity was critical in accurately characterizing ICU workload, resulting ICU discharge decisions, and ultimately patient outcomes and hospital costs. Our findings suggest that (1) ICUs need to track the changes in patient acuity in addition to census, in order to use such information to prevent reaching high census with high acuity patients and (2) future studies (both empirical and analytical) of ICU workload should take patient acuity into account in ICU workload measures. Lastly, using a simulation study, we show how high-census/high-acuity workload can be prevented by reducing seasonality in the volume and acuity of patient arrivals.